Starch from grains can be converted to ethanol by biochemical pathways. Most corn is dry milled—that is, the grain is ground to a meal, and then the starch from the grain is hydrolyzed by enzymes to glucose. The 6-carbon sugars are then fermented to ethanol by natural yeast and bacteria. The fermented mash is separated into ethanol and residue by distillation (Figure 2-2). The residue can be marketed wet as a dairy or cattle feedstuff or as dried distillers grain with solubles (DDGS) as a dairy, cattle, swine, and poultry feedstuff (Schwietzke et al., 2008). The theoretical yield of converting corn starch to ethanol is 112 gallons per dry ton (Patzek, 2006). A survey of U.S. ethanol plants conducted in 2008 reported average ethanol yield of 100 gallons per dry ton (Mueller, 2010).
Products and Coproducts
DDGS is a coproduct of grain ethanol production. Starch from grains is fermented to ethanol and the remaining protein, oil, yeast, minerals, and fiber form the coproduct DDGS, which is mostly used as an animal feedstuff (Nichols et al., 2006). For every bushel of corn grain used for ethanol production, about one-third comes out as DDGS, one-third as ethanol, and one-third as carbon dioxide (CO2). The wet-mill process also produces corn oil and high-fructose corn syrup as coproducts.
Distribution and Use
Many biofuels, including ethanol, are more soluble in water than petroleum-based fuels, requiring biofuels to be stored and handled more carefully to avoid water contamination. If